In order to regulate emissions from the exhaust system of a vehicle, exhaust gas treatment systems are employed in an engine's exhaust passage. In some examples, the exhaust gas treatment system may include a selective catalytic reduction (SCR) system to reduce nitrogen oxide (NOx) emissions from the exhaust. Vehicles equipped with the SCR system inject a reducing agent, such as an aqueous solution of urea ((NH2)2CO), into the exhaust passage upstream of a SCR catalyst. The urea solution upon thermal decomposition in the exhaust passage forms ammonia (NH3), which is then adsorbed onto the catalyst surface. NOx gases in the exhaust passage react with the adsorbed ammonia to form nitrogen (N2) and water (H2O), for example.
The aqueous urea solution may be stored in an exhaust fluid storage tank on-board the vehicle and monitored, so that appropriate default action can be taken if insufficient solution is maintained, or if improper diluents are added. Tank level sensors may be utilized to detect the quantity of urea remaining in the tank in order to aid monitoring of the SCR system. When the level of urea solution is low, a warning may be indicated to the driver. Likewise, changes in the level may be used in combination with other data to detect a quality of the solution. However, during certain vehicle operating conditions, when the exhaust fluid tank is subjected to acceleration and/or inclination, the level sensor may indicate incorrect liquid level measurements.
One example approach to address incorrect level measurements by the level sensor is provided by Minezawa et al. in US 2013/0055700. Therein, an output of a urea level sensor is passed through a low-pass filter having a time constant based on engine speed. However, the inventors herein have identified potential issues with such an approach. For example, Minezawa's approach assumes distortions in level sensor measurements are correlated to vibrations caused by speed, and does not account for loss of signal due to inclination of the exhaust fluid storage tank or surface ripple, which may occur when the vehicle is in motion and when the vehicle is at rest. For example, certain vehicle operating conditions, which cause sloshing or inclination of the tank, may increase or decrease the height of the liquid above the sensor even though there may be no change in the total quantity of liquid in the tank. This sloshing and/or ripple does not just cause measurement errors, but may actually cause a lack of signal when using an ultrasonic sensor configuration. The loss of signal can drastically impact the estimate, even if filtered with a low pass filter. Further, such loss of signal situations have little to no correlation with engine speed.
In one example, the above issues may be at least partly addressed by a method, such as a method for an exhaust selective catalytic reduction system. The method may comprise generating an ultrasonic signal in an exhaust fluid tank of a vehicle and indicating fluid level based on a filtered reflection of the ultrasonic signal, the reflection filtered based on a strength or other quality of the received reflection. For example, strength of the reflected signal may be below a threshold due to an inclination of the urea surface. Consequently, the reflected signal may have a lower quality that can be used to differentiate between large changes in sensor output that are cause by mis-reflections versus large changes in sensor output cause by large changes in the fluid level itself. In this way, by filtering the level measurement from the sensor based on quality of the reflected signal, as well as based on the level measurements themselves, more accurate level measurements may be obtained.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.